Certain metals, often referred to as shape memory alloys, undergo a phase transformation upon a change in temperature. These alloys are characterized by memory of a mechanical configuration imposed upon the alloy during its austenite phase. At a lower temperature, the alloy's martensite phase allows the alloy to be relatively easily deformed into a particular shape. If, when in that particular shape, the alloy is heated to a temperature in which the alloy undergoes a phase transformation from martensite phase to austenite phase, the memory effect of the alloy is manifested by a return to the shape ordinarily imparted upon the alloy in its austenite phase.
Many different types of actuators have been proposed using shape memory alloys. A typical actuator construction is to use thin wires of the shape memory alloy. The actuation is caused by the elongation and contraction of the wires. Also, these actuators typically use resistive methods for heating the alloys and air/water convection methods to cool the alloys.
An example of an actuator using shape memory alloy is found in U.S. Pat. No. 5,396,769 to Brudniki. Brudniki shows two capstans mounted on a shaft which is supported in a framework with each capstan capable of rotating the shaft. Two separate lengths of shape memory wire are wrapped around each capstan to form a winding around that particular capstan. One wire is wrapped in a pre-stretched state and the other is not. When heated, one wire performs work in one direction, and when the other wire is heated the action is reversed.
Another example is found in U.S. Pat. No. 5,127,228 to Swenson. Swenson shows a rotary actuator using shape memory alloy. The actuator is made of two concentric tubular shape memory alloy members torsioned along their longitudinal axis with ends constrained relative to each other. One end of the actuator is constrained while the other is the output. A heater is located inside the inner shape memory alloy member, and a heater is located on the outside of the outer shape memory alloy member. The unconstrained end is caused to rotate between positions by applying current to the appropriate heater.
A mechanical actuator is taught by U.S. Pat. No. 4,553,393 to Ruoff. The mechanical actuator is constructed using a plurality of shape memory actuator elements in parallel to control the amount of actuating force. The actuating elements may vary in stiffness according to a binary relationship. The cooling time of the actuator elements may be employing Peltier junction cooling assemblies in the actuator.
Still another actuator using shape memory alloy is taught in U.S. Pat. No. 4,700,541 to Gabriel et al. Gabriel shows an electrically controlled shape memory alloy actuator made of a shape memory wire which is torsioned along its longitudinal axis and with its ends constrained against movement. A lever is attached to a wire at a desired point other than at a wire end. Electrical connections spaced along the wire define different sections along the wire. The wire and attached control element are made to rotate by selectively apply voltages to the different sections of the wire to heat the individual sections.
A problem, however, with most types of these actuators is that the frequency of operation is not very high. The frequency being the amount of cycles over a given time that the actuator performs, and a cycle defined as the total of the movement of the actuator from the initial state to an intermediary state and back to the first state. One cause for the low frequency of operation is the length of time required to cool the memory shape alloy.
Another problem associated with these types of actuators is that geometries other than thin wires of the shape memory alloy cause the electrical resistance of the shape memory alloy to be very low. As such, to heat the alloy in such shapes using resistive heating requires a large current. Thus, unless an external heater is employed, a large power system would likely be needed to supply the actuator with enough current to heat the alloy sufficiently to cause a phase transformation.